1. Field of the Invention
The present invention relates to a semiconductor circuit, and particularly to a semiconductor circuit suitable for use in hybrid ICs (HIC: Hybrid Integrated Circuits) for CATV (cable television).
2. Description of the Related Art
Wideband amplifiers used for amplifying and relaying signals in CATV systems must both provide extremely low-distortion amplification to avoid deterioration of image quality and maintain performance above a predetermined level even under severe outdoor conditions. Wideband amplifiers conventionally used in a CATV system have therefore employed circuits to compensate fluctuations in the gain characteristic caused by changes in ambient temperature. However, variations in ambient temperature affect not only the gain of the amplifier circuit, but the distortion characteristic as well. Each element making up the semiconductor circuit generally has a characteristic whereby distortion increases with rises or falls in temperature from a particular fixed temperature. Although there are circuits that compensate gain with changes in the ambient temperature, no circuits exist for compensating the deterioration in distortion characteristics that accompanies variations in ambient temperature.
In consideration of the problems of the above-described prior art, the present invention was realized with the object of providing a semiconductor circuit that can compensate distortion in the event of changes in ambient temperature.
The object of the present invention is achieved by a semiconductor circuit comprising an amplifying circuit that amplifies an alternating-current signal and outputs an amplified signal, and a compensating circuit that compensates a distortion of the amplified signal with changes in ambient temperature.
In the present invention, a compensating circuit is typically provided that combines a thermo-sensitive resistance element in which resistance changes in accordance with the ambient temperature with a positive temperature characteristic and another thermo-sensitive resistance element in which resistance changes in accordance with ambient temperature with a negative temperature characteristic to compensate variation in distortion of signals outputted from the amplifying circuit that is caused by changes in ambient temperature. If current flowing at a reference temperature is set at a minimum when combining these thermo-sensitive resistance elements having positive and negative temperature characteristics, the circuit current increases as the ambient temperature falls below the reference temperature, and moreover, increases as the ambient temperature rises above the reference temperature.
Here, distortion in amplification generally decreases when the circuit current of the amplifying circuit increases and increases when the circuit current decreases, and distortion resulting from increase in the circuit current therefore decreases when the ambient temperature changes if changes in distortion arising from changes in temperature of the elements themselves are ignored. Thus, if the reference temperature is set as the temperature at which distortion of each of the elements making up the semiconductor circuit is a minimum, the increase in distortion of the elements themselves resulting from changes in the ambient temperature is canceled by the decrease in distortion resulting from the increase in the circuit current, thereby compensating distortion resulting from changes in ambient temperature.
In the present invention, thermistors can be used as the thermo-sensitive resistance elements.
If thermistors are provided on the input side of the amplifying circuits as the thermo-sensitive resistance elements having a negative temperature characteristic, the Q factor (the quality factor) decreases to the extent that the resistance of the thermistors rises and increases to the extent that the resistance decreases, and the Q factor of the amplifying circuit thus increases as the ambient temperature rises and decreases as the ambient temperature falls. Here, the Q factor is a factor indicating the resonance level. In a semiconductor device, the gain slope becomes moderate as the ambient temperature rises and becomes steep as the ambient temperature falls and gain increases. Arranging a thermistor having a negative temperature characteristic on the input side of the amplifying circuit therefore causes fluctuation in the Q factor with respect to the ambient temperature to be canceled by fluctuation in the gain characteristic with respect to ambient temperature of the gain slope, whereby the slope characteristic of the gain slope is fixed regardless of changes in the ambient temperature.
The above and other objects, features, and advantages of the present invention will become apparent from the following description based on the accompanying drawings which illustrate examples of preferred embodiments of the present invention.